This invention relates to a capacitive keyboard for data processing equipments, having a variable capacitor associated with each key.
There are known capacitive keyboards, wherein each key comprises a variable capacitor with two armatures, one fixed and one movable with the movement of the key, with the interposition of a thin dielectric layer.
The recognition of the state of actuation of the capacitive keys and the generation of the corresponding codes are currently operated by means of devices known as KBE (KeyBoard Encoders), that require the connection of the armatures of the individual capacitive keys according to the rows and columns of a matrix.
In some known capacitive keyboards the fixed armatures and the movable armatures are separately connected by rows and columns respectively; while the connection of the fixed armatures obtained from a printed circuit-board is quite simple and inexpensive, the connection between the movable armatures, and their connection to the printed circuit board carrying the fixed armatures and the KBE is much more complicated.
In another known keyboard both armatures of the variable capacitors are obtained from the printed circuit board, by dividing in two halves the surface of the printed circuit board underlying each key, the variation of capacitance between the armatures effected by the displacement of a movable armature facing both halves. In this keyboard however, with the same dimensions of the keys, the variation in capacitance is considerably inferior as compared to the preceeding solution. Furthermore the connection of the two halves corresponding to each key by rows and columns unavoidably requires the crossing of the conductors, requiring the use of printed circuits with two faces considerably more expensive than the printed circuits with only one face.
In key boards using a polarized dielectric or electret, at the depression of a key, it is necessary to detect the pulses generated between the two armatures of the variable capacitors by electrostatic induction. Said pulses have opposite polarity on the two armatures of the variable capacitors, preventing the normal organization by rows and columns for the coding. In a known keyboard (U.S. Pat. No. 3,653,038), the fixed armatures of the variable capacitors are also divided in two halves, of which one is connected to the row and the other to the column system, while the movable armature is set at a common earth potential. However also this keyboard requires a two sided printed circuit board, because the row and column connections of the two halves of each key must cross.
The main object of the invention is to provide a keyboard having the advantages of reliability of the capacitive and electret keyboards and adapted to be manufactured at a low cost. This is achieved by drastically reducing the number of the component pieces and by the use of printed circuits with only one face.